The gypsum by-products derived from the production of phosphoric acid by the wet method and having the formula CaSO.sub.4.2H.sub.2 O still constitute little used products which pose serious ecological problems. The production of phosphogypsum in phosphoric acid plants employing the wet method (action of sulfuric acid on phosphate rock) is increasing by hundreds of tons daily, and it is no longer possible for ecological reasons to continue dumping it into the sea or into rivers as has hitherto been the case. Moreover, storage of this product in dumps, as is currently practiced by numerous firms, also poses problems which are insurmountable in the long term.
Much research has been directed to the study of phosphogypsums in an effort to process from them raw materials having an intrinsic value.
Most of this research has been directed to the preparation, by more or less extensive dessication, of different types of plasters such as the semi-hydrates or more recently the hemi-hydrates, CaSO.sub.4.1/2H.sub.2 O, of the .alpha. hard-plaster variety or the .beta. flowable plaster of Paris variety and the anhydrous rehydratable CaSO.sub.4 varieties such as hydraulic plasters.
On the other hand, there has been much less research into the anhydrous forms of CaSO.sub.4 which are non-rehydratable after calcination, as these products were considered as having virtually no industrial importance.